This was a connected world, not closed in on itself, as we can infer from the rapid spread from the Atlantic to the Black Sea of elements such as La Tène art. In the ongoing debate about who the Celts were, or if we can even speak of Celts, I consider them as the groups who wove that network of contacts linking large parts of continental Europe. Their identity is embodied in a series of elements shared to a greater or lesser degree, such as certain features of material culture, their ideological, religious, and institutional universe, and related languages. Of course, this ‘cultural package’ was not homogeneous, immutable, or shared in its entirety. These common elements were also perceived by Greco-Roman observers with little nuance, and they homogenized these peoples under the terms Keltoi, Galatai, Galli, or Celtae, although most of them probably did not assume, or even consider, such a macro-ethnic identity.
However, we do know endogenous Celtic ethnonyms of limited use mainly through the classical written sources. They are also attested in epigraphy or monetary legends. This is evidence of the existence of ethnic identities to which certain individuals and groups in continental Europe would have felt they belonged. There is some): e.g. Senones, Cenomani, or Lingones, which have traditionally been associated with Celtic migrations to Italy.
